1886. ] STRUCTURE OF THE OVUM IN THE DIPNOI. 507 
division of the ovum itself into two zones by a definite membrane 
would seem to be an absurdity; in the second place, no membrane 
was evident in preparations where the ovum was broken. It might be 
expected that when the ovum was broken in cutting, the membrane, 
being presumably of a different hardness to the egg-protoplasm, 
would project from the cut surface ; in no instance, however, did the 
broken surface show any indication anywhere of a membrane. 
The line of division between the two yolk-zones presented the 
appearance in my preparations of an absolute break ; the protoplasm 
was perfectly transparent, and, being unaffected by the staining- 
reagent, was invisible. 
I did not notice this differentiation of the yolk in all the large ova 
visible in my sections. In some ova, which were full of yolk, and of 
equal size with those just referred to, there was no trace of any such 
specialization into a peripheral and central zone; in these cases the 
yolk was uniform throughout. Such ova were to be found not only 
in the same ovary, but in the same section with the ova which dis- 
played a differentiation of the yolk. This circumstance renders it 
improbable that the effect of reagents has caused the yolk to acquire 
a uniform appearance. 
A comparison of the two kinds of ova has led me to the conclusion 
that the ova in which there is a specialization of the yolk are nearly 
mature, while those in which the yolk is uniform are degenerating ova. 
Another matter relates to the structure of ege-membranes and 
their homologies, where I have to make a correction. 
In my former paper I have referred to the presence in compara- 
tively young ova of a vertically striate membrane lying within the 
vitelline membrane (loc. cit. p. 273, pl. xxviii. fig. 1, 2.7; pl. xxix. 
fig. 2, 2.7). This, it now appears to me, is not the equivalent of the 
inner of the two membranes which surround the Teleostean ovum’. 
The early disappearance of this membrane and its general structure 
granular and with no distinct line of separation from the subjacent 
egg-protoplasm) were against such an interpretation ; I now identify 
it with more confidence with a specialized layer of the egg-protoplasm 
described by Brock in Alburnus lucidus, Salmo fario, and Perca 
fluviatilis, and by Owsiannikow in Acerina vulgaris. Brock has 
figured this layer (the ‘‘ Zonoidschicht”’ of His, the “helle Rand- 
schicht”’ of Gegenbaur) in Alburnus lucidus (Morph. Jahrb. Band iv. 
pl. xxviii. fig. 12, fg.), where it is more complicated than in Lepido- 
siren and consists of two layers—an inner homogeneous and an outer 
vertically striate layer. 
ConTENTS OF THE OvARyY OF PROTOPTERUS. 
The following is a detailed account of the structure and develop- 
ment of certain bodies in the ovary which have already been partly 
described in my former paper; they are nearly as numerous as the 
ordinary ova. 
1 Of.J. T. Cunningham, “ On the Mode of Attachment of the Ovum of Osmerus 
perlanus,” P.Z.S. 1886, pt. iii. p. 292, pl. xxx. fig. 4, z.7.i, and other memoirs. 
